1. Field of the Invention
The present invention relates to a measurement apparatus and a measuring method for a pattern using a charged particle beam. More specifically, the present invention relates to a pattern measurement apparatus and a pattern measuring method capable of distinguishing between lines and spaces provided at even intervals in a spaced line-and-space pattern.
2. Description of the Prior Art
Measurement using a scanning electron microscope has heretofore been applied to a line-width measuring method of a pattern. Here, a scanning electron microscope is configured to scan an electron beam scanning range by irradiating incident electrons, to acquire secondary electrons emitted from a sample by using a scintillator, to convert a quantity of electrons thus acquired into luminance, and to display a surface image of the sample on a display device.
In the case of managing characteristics of a semiconductor device by use of this scanning electron microscope, it is a general practice to check whether or not a line width of a pattern is formed in the size within a design standard. The management of the pattern line width is typically executed in accordance with the following procedures. Specifically, after displaying a predetermined range of a pattern formed on a photomask on a display device, an electron beam is focused and irradiated on a measurement point in the displayed range to acquire a waveform of luminance distribution based on secondary electrons reflected from the measurement point. Then, a high-level width in the waveform of luminance distribution is determined as a line width. A judgment is made as to whether or not this line width falls within an acceptable error range. If the line width is within the acceptable error range, a subsequent process is executed. In contrast, if the line width is out of the acceptable error range, the photomask is sent back to a process for forming the pattern.
In this way, the line-width measurement of the pattern is important in the manufacturing process of the semiconductor device, and various methods for accurately measuring the line width have been disclosed.
A position where a slope of luminance corresponding to a quantity of secondary electrons becomes the maximum is generally defined as an edge position of the pattern, whereas Japanese Patent Application Laid-open Publication No. Hei 5(1993)-296754 discloses an edge detection method of determining, as an edge position, a position where a secondary electron signal becomes the minimum.
Meanwhile, Japanese Patent Application Laid-open Publication No. 2005-195361 discloses a method of calculating an average line width and an average space width from auto-correlation values between an original image obtained by differentiating once luminance information of a line-and-space pattern and a shifted image in the x direction.
As described above, the line-width measurement of the pattern with a scanning electron microscope employs the method of determining the position where the slope of luminance becomes the maximum as the edge position or the method of determining the position where the secondary electron signal becomes the minimum as the edge position.
However, application of these methods of detecting the edge position to the line-and-space pattern turns out to cause the following problem.
When the width of a line pattern is approximately equal to the width of a space pattern, it is possible to detect edges, but it is difficult to judge whether intervals between the edges constitute the line pattern or the space pattern.
In contrast, it is possible to judge whether the intervals constitute the line pattern or the space pattern by considering the luminance (tone) information of the pattern. For example, the tone of a line pattern 63a is usually higher than the tone of a space pattern 62a as shown in FIG. 1A. It is therefore possible to judge that the pattern 63a between edges 61 having the higher tone is the line pattern.
Nevertheless, there are various types of constituent material or ranges of film thickness that may cause a line pattern 63b and a space pattern 62b not to have a difference in the tone as shown in FIG. 1B or a case in which the tone relation between a line pattern 63c and a space pattern 62c is reversed as shown in FIG. 1C. In such cases, there is a risk of misjudging whether a portion between the edges constitutes the line pattern or the space pattern.
There have been no reports concerning techniques for distinguishing between the line pattern and the space pattern in the line-and-space pattern in which the line width and the space width are set almost equal to each other.